Organ culture of the fetal mouse palate for screening the developmental toxicity of chemicals: a validation study.

Using in vitro organ culture of the fetal mouse palate in a chemically defined serumless medium, the toxicity of 24 chemical compounds was investigated. Explanted palates of day-12.5 mouse fetuses were exposed for 72 h in vitro to various concentrations of each chemical, and the fusion rate and growth parameters were compared between the experimental group and respective controls. The average rate of palate fusion was 84% in vehicle controls. For compounds that are teratogenic in experimental animals in vivo, the fusion rates of palatal shelves decreased as the concentration of the test chemicals increased, showing a dose-dependent relationship. Palate fusion was inhibited by 11 of the 15 in vivo teratogens, and the predictability of in vivo developmental toxicity in this culture system was 73%. Cyclophosphamide itself did not inhibit the growth and fusion of explanted palates, but supplementation of hepatic S-9 fraction and cofactors for a monooxygenase system converted it to a toxic substance, as was shown in other in vitro systems. The 50% inhibitory concentration (IC50) value calculated based on the fusion rate was also found to be a useful parameter for evaluating the developmental toxicity of drugs. The teratogenic risk in the human fetus could be assessed by comparing the minimal toxic concentrations of the test compound on cultured palates with the maximal plasma level in pregnant women under therapeutic conditions and with the plasma concentrations when its minimal teratogenic dose is given to pregnant mice. This organ culture system of the fetal palate should be useful for screening the developmental toxicity of drugs and other environmental agents, and its value should increase when it is used in combination with other battery test systems.

Palatal shelf movement during palatogenesis: a fate map of the fetal mouse palate cultured in vitro.

Day-13 fetal mouse palates (plug day=day 0) were labeled with carbon particles at various sites of palatal shelves and cultivated in a chemically defined medium for up to 48 h. During the culture period, the bilateral palatal shelves came in contact and fused with each other, which simulated in vivo palatogenesis. The carbon study revealed that at the midpalatal region, the medial edge of the palatal shelf elevated to the horizontal plane, elongated toward the midline, and made contact with the medial edge of the opposing shelf. On the other hand, near the anterior and posterior ends of the shelf, some new tissue was formed at the medial edge of the shelf by remodeling and this newly formed tissue took part in palatal fusion. The results of the present study indicate that during mouse palatogenesis, the anterior and posterior regions of the palatal shelf behave differently from the midpalatal region. It seems that in the fetal mouse palate, the midpalate closes mainly by means of rotation and medial elongation of the shelf, whereas the anterior and posterior parts of the palate close mainly by tissue remodeling of the medial edge and partly by medial elongation of the shelf.